Mammalian cytoplasmic actins are the products of at least two genes and differ in primary structure in at least 25 identified positions from skeletal muscle actins

Fimbrin is a cytoskeletal protein that crosslinks F-actin in vitro

Fimbrin is a cytoskeletal protein associated with microfilaments in microvilli, microspikes, stereocilia, membrane ruffles, and cell--substratum attachment sites. Fimbrin purified from intestinal epithelial cell brush borders was found to be a monomeric protein of molecular weight 68,000. In a sedimentation assay, fimbrin bound to F-actin in a salt-dependent manner, with binding being optimal in 30 mM KCl and inhibited in greater than 100 mM KCl. In 50 mM KCl, which allows efficient polymerization of actin, the interaction was stabilized by the presence of polyethylene glycol. Under these conditions, binding was unaffected by the inclusion of up to 5 mM Ca2+ but was inhibited by greater than 0.5 mM Mg2+. Electron microscopy revealed that fimbrin crosslinked F-actin into relatively straight bundles with shorter bundles being formed at high fimbrin-to-actin ratios. The results suggest that fimbrin crosslinks F-actin in such a way as to confer some rigidity on the bundle formed. This proposed function for fimbrin is consistent with its in vivo localization in straight, highly organized, microfilament bundles such as microvilli, microspikes, and stereocilia.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

Gene switching in myogenesis: differential expression of the chicken actin multigene family

We described the construction of an alpha-actin complementary deoxyribonucleic acid (cDNA) clone, pAC269 [Schwartz, R. J., Haron, J. A., Rothblum, K. N., & Dugaiczyk, A. (1980) Biochemistry 19, 5883], that was used as a hybridization probe in the current investigation to examine the induction of actin messenger ribonucleic acid (mRNA) during myogenesis. A Tm difference of 10-13 degrees C between skeletal muscle alpha-actin and nonmuscle beta- and gamma-actin mRNAs and pAC269 allowed us to establish the highly stringent hybridization conditions necessary to measure separately the content of alpha-actin mRNA and beta- and gamma-actin mRNA during muscle development in culture. We observed low levels of alpha-actin mRNA (approximately 130 molecules/cell) in replicating prefusion myoblasts. The vast majority of actin mRNA (2000 molecules/cell) present at this stage was accounted for by beta- and gamma-actin mRNA. Beginning at myoblast fusion, alpha-actin mRNA accumulated and within 30 h reached a level 270-fold greater than that observed in the undifferentiated state. At 95 h in culture when myotube formation was completed, alpha-actin content was at its peak (36 000 molecules/nucleus). Conversely, beta- and gamma-actin mRNA content began to decline at the beginning of fusion, and by the end of myotube formation beta- and gamma-actin mRNAs were undetectable by our techniques. A rapid depression of alpha-actin mRNA levels was observed after 95 h in the absence of cell death. At 6 days after the initiation of myotube formation, the content of alpha-actin mRNA was reduced by 80% in comparison of peak values and remained at that level. The switching of actin mRNA species was inhibited in myoblasts treated with bdU. The accumulation of alpha-actin mRNA and the disappearance of beta- and gamma-actin mRNA were observed following the reversal of the bdU block and coincident with the onset of myoblast fusion. We found that the expression of actin genes within the actin multigene family is switched in myogenesis through a strict developmental pattern.

Brain actin synthesized in vitro undergoes two different and sequential posttranslational modifications

The have studied the posttranslational processing of actin molecules synthesized in a cell-free system. The results of these experiments indicate that during the in vitro synthesis of the actins from rat brain the primary translational products undergo two different and sequential posttranslational modifications. These modifications are accompanied by slight changes in the isoelectric points of the proteins and can be detected by isoelectric focusing analysis. The same posttranslational modifications can be detected during the in vitro synthesis of chick embryo skeletal muscle actin. The evidence presented suggest that the first posttranslational modification may correspond to the methylation of a histidine residue, and the second modification most likely corresponds to the acetylation of the NH(2)-terminal amino acid residues of actin molecules.

Sea urchin tube feet: unique structures that allow a cytological and molecular approach to the study of actin and its gene expression

Actin is the major extractable protein component from the tube feet of four different species of sea urchin: Arbacia punctulata, Strongylocentrotus purpuratus, Strongylocentrotus droebachiensis, and Diadema setosum. Actin made up as much as 60% of the total Coomassie Blue-staining material after SDS polyacrylamide gel electrophoresis and densitometer analysis. Two-dimensional gel electrophoresis resolved two, and possible three, species of actin for each sea urchin of which the dominant component was analogous to the beta form in vertebrates. In a cell-free system from rabbit reticulocytes, total RNA from tube feet stimulated the synthesis of one protein that represented 80% of the total methionine incorporation, migrated with the properties characteristic of actin in a two-dimensional gel system, and on proteolysis yielded fragments identical to purified rabbit actin. The mRNAs from the tube feet of two divergent species of sea urchin, Arbacia punctulata and Strongylocentrotus purpuratus, synthesized actins differing by less than 0.02 pH unit for each isospecies 90% of the DNA copied from tube foot RNA by reverse transcriptase represented a highly abundant sequence class judged by copy DNA(cDNA)-RNA excess hybridization. At least two-thirds of this class represented a low-complexity component, with a Rot1/2 about three times that expected for actin messenger RNA. The remarkable degree of conservation of the actin protein is reflected in concomitant conservation of the protein-coding nucleotide sequences of the messenger RNA, which has allowed the use of a cDNA probe to isolate actin sequences from a human phage library.

Actin acetylation in Drosophila tissue culture cells

In a permanent cell line derived from Drosophila embryos, cytoplasmic actin is produced as an unstable precursor, which is subsequently converted to a stable form. This conversion results in a reduction in isoelectric point, with no apparent change in molecular weight. The conversion involves an enzymatic acetylation, and results in an insensitivity to aminopeptidase digestion, suggesting N-terminal blockage. Both the acetylated and unacetylated actins can participate in the assembly of F-actin, but with different efficiencies.

Evidence for a species of nuclear actin distinct from cytoplasmic and muscles actins

Nuclear actin (protein BJ) has been isolated from the chromatin of Novikoff hepatoma ascites cells and purified to homogeneity by selective extraction, Sepharose CL-6B chromatography, and preparative polyacrylamide gel electrophoresis. A comparison of nuclear and cytoplasmic actins from Novikoff hepatoma cells and rabbit muscle actin was made by amino acid analysis, isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two-dimensional peptide mapping procedures. By these criteria, all of the proteins compared are actins, but each is chemically distinct. It was concluded, therefore, that nuclear actin is similar to, but not identical with, cytoplasmic actin isolated from Novikoff hepatoma cells. A striking similarity in peptide charge and migration as shown by peptide map analysis was observed for nuclear and rabbit skeletal muscle actins. This may indicate that nuclear actin has the capacity for contractile function. In addition, the actins synthesized in Novikoff hepatoma cells may results from more than two structural genes.

Nucleotide and corresponding amino acid sequences encoded by alpha and beta tubulin mRNAs

Most of the mRNA sequences coding for alpha and beta tubulin in embryonic chick brain have been determined by sequencing of cloned cDNA copies of these mRNA copies of these mRNAs. From a 1,682-base pair cDNA sequence we have deduced the entire protein sequence for beta tubulin. For alpha tubulin, all but about 38 N-terminal amino acids have been deduced from the cDNA sequence. Although tyrosine has previously been shown to be post-translationally added to the C-terminus of alpha tubulin by a specific ligase, we conclude that the primary post-translational even must be the removal, not the addition of tyrosine because a terminal tyrosine is encoded by the mRNA.

Skeletal muscle actin mRNA. Characterization of the 3' untranslated region

Plasmids p749, p106, and p150 contain cDNA inserts complementary to rat skeletal muscle actin mRNA. Nucleotide sequence analysis indicates the following sequence relationships: p749 specifies codons 171 to 360; p150 specifies codons 357 to 374 together with 120 nucleotides of the 3'-non-translated region; p106 specifies the last actin amino acid codon, the termination codon and the entire 3' non-translated region. Plasmid p749 hybridized with RNA extracted from rat skeletal muscle, cardiac muscle, smooth (stomach) muscle, and from brain. It also hybridizes well with RNA extracted from skeletal muscle and brain of dog and chick. Plasmid p106 hybridized specifically with rat striated muscles (skeletal and cardiac muscle) mRNA but not with mRNA from rat stomach and from rat brain. It also hybridized to RNA extracted from skeletal muscle of rabbit and dog but not from chick. Thermal stability of the hybrids and sensitivity to S1 digestion also indicated substantial divergence between the 3' untranslated end of rat and dog skeletal muscle actins. The investigation shows that the coding regions of actin genes are highly conserved, whereas the 3' non-coding regions diverged considerably during evolution. Probes constructed from the 3' non-coding regions of actin mRNAs can be used to identify the various actin mRNA and actin genes.

Actin typing on total cellular extracts: a highly sensitive protein-chemical procedure able to distinguish different actins

Based on the finding that the amino-terminal tryptic peptide of actin is a reliable marker for actin divergence, we describe in detail a highly sensitive protein-chemical procedure for actin typing. The method is performed on non-radioactivity labeled cells and tissues and six actins can be identified unambiguously in warm-blooded vertebrates. The method is quantitative and gives directly the ratio of the different actions in the specimens. It does not require previous purification of actin and can be used on total cellular extracts without any prior fractionation. The procedure can be extended to actins not previously characterized by amino acid sequence analysis and makes certain predictions possible about the partial amino acid sequences of the amino-terminal tryptic peptides, mostly sufficient for a correlation with DNA sequences derived from cloned actin genes. This is done as an example for the cytoplasmic action present in Schneider L-2 Drosophila melanogaster cells. Although the method is currently used routinely on 10(5) cells, modifications are discussed, which should allow the analysis to be performed with even higher sensitivity.

Permanently proliferating rat vascular smooth muscle cell with maintained expression of smooth muscle characteristics, including actin of the vascular smooth muscle type

Cells of an established clonal line (RVF-SMC) derived from rat vena cava are described by light and electron microscope methods and biochemical analysis of the major proteins. The cells are flat, and they moderately elongate and form monolayers. They are characterized by prominent cables of microfilaments bundles decoratable with antibodies to actin and alpha-actinin. These bundles contain numerous densely stained bodies and are often flanked by typical rows of surface caveolae and vesicles. The cells are rich in intermediate-sized filaments of the vimentin type but do not show detectable amounts of desmin and cytokeratin filaments. Isoelectric focusing and protein chemical studies have revealed actin heterogeneity. In addition to the two cytoplasmic actins, beta and gamma, common to proliferating cells, two smooth muscle-type actins (an acidic alpha-like and a gamma-like) are found. The major (alpha-type) vascular smooth muscle actin accounts for 28% of the total cellular actin. No skeletal muscle or cardiac muscle actin has been detected. The synthesis of large amounts of actin and vimentin and the presence of at least three actins, including alpha-like actin, have also been demonstrated by in vitro translation of isolated poly(A)+ mRNAs. This is, to our knowledge, the first case of expression of smooth muscle-type actin in a permanently growing cell. We conclude that permanent cell growth and proliferation is compatible with the maintained expression of several characteristic cell features of the differentiated vascular smooth muscle cell including the formation of smooth muscle-type actin.

Organization of actin gene sequences in the sea urchin: molecular cloning of an intron-containing DNA sequence coding for a cytoplasmic actin

Southern transfer and solution hybridization experiments, using as probe a DNA fragment that encodes for Drosophila actin, demonstrate cross hybridization to DNA from the sea urchin Strongylocentrotus purpuratus. Recombinant DNA clones that contained sea urchin genomic DNA fragments were constructed and screened for the presence of actin-encoding DNA sequences by colony hybridization with the Drosophila actin sequence. Two different putative actin-encoding clones were identified and were shown to specifically hybridize actin-encoding mRNA from a complex mRNA population. Southern blot hybridization experiments with both the Drosophila actin sequence and one of the cloned sea urchin sequences, in conjunction with solution hybridization data, suggest an actin gene copy number of 5-20 per haploid genome. DNA sequence analysis of one of the cloned sequences indicates that this fragment codes for a cytoplasmic form of actin and contains an intervening sequence of at least 200 nucleotides beginning immediately after amino acid 121 in the protein sequence.

Putative actin genes in the macronucleus of Oxytricha fallax

Previous work has shown that the macronuclear DNA of the hypotrichous ciliate Oxytricha fallax is arranged as short achromosomal pieces, 22 to 0.5 kilobase pairs (kb) in length. Micronuclear DNA has a typical chromosomal organization. Macronuclear DNA is derived from micronuclear DNA through a process of polytene chromosome fragmentation with a resultant decrease in DNA sequence complexity. Three putative actin genes have been identified in macronuclear DNA by using a cloned yeast actin gene as a hybridization probe. A restriction fragment of the yeast gene containing both actin coding and noncoding DNA hybridizes strongly to two macronuclear DNA pieces, 1.6 and 1.4 kb in length, and weakly to a 1.2-kb piece. The entire 1.6-kb piece has been cloned in plasmid pBR322 and the resulting recombinant plasmid has been designated pOfACT(1.6). The 1.6-kb pOfACT(1.6) insert hybridizes only to those restriction fragments of the yeast actin gene containing actin coding sequences. When hybridized to macronuclear DNA under conditions that allow the yeast probe to hybridize to all three macronuclear pieces, the pOfACT(1.6) insert hybridizes only to the 1.6-kb piece. Under less stringent conditions the insert also hybridizes to the 1.4-kb piece, but it shows no hybridization to the 1.2-kb DNA. The three macronuclear pieces homologous to the yeast actin gene thus differ in sequence and are interpreted as a related family of actin genes. Each of these pieces could accommodate an actin coding sequence, which in yeast, Dictyostelium discoideum, and Drosophila melanogaster is 1.1 kb, and an additional 0.1-0.5 kb of noncoding DNA.

Isolation and sequence of the gene for actin in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is known to contain the highly conserved and unbiquitous protein actin. We have used cloned actin sequences from Dictyostelium discoideum to identify and clone the actin gene in yeast. Hybridization to genomic fragments of yeast DNA suggest that there is a single actin gene in yeast. We have determined the nucleotide sequence of that gene and its flanking regions. The sequence of the gene reveals an intervening sequence of 309 base pairs in the coding sequences at the 5' end of the gene. The existence and location of the intervening sequence was verified by using the dideoxy chain termination technique to determine the sequence at the 5' terminus of the actin mRNA. The similarity of the splice junction sequences in this gene to those found in higher eukaryotes suggests that yeast must possess a similar splicing enzyme.

A study of invertebrate actins by isoelectric focusing and immunodiffusion

Actin isolated from various invertebrate phyla comigrates with the beta-form of vertebrate smooth muscle actin. However, invertebrate actins are not identical, since antibodies to insect-actin will not crossreact with the other species.

Vegetative Dictyostelium cells containing 17 actin genes express a single major actin

Although actin is highly conserved between different eukaryotic species, six tissue-specific actins have been characterised in higher vertebrates by complete amino acid sequence analysis (two cytoplasmic actins, two smooth muscle actins and two sarcomeric actins). Their tissue specificity suggests they may differ in some important although unknown physiological property. Actin expression in lower eukaryotes seems to be a simpler process than in higher eukaryotes since biochemical experiments have indicated only one major type in purified preparations from various species. However, Firtel et al. have isolated several recombinant plasmids containing sequences of Dictyostelium discoldeum DNA complementary to actin messenger RNA and have suggested that this unicellular slime mould may have 17 actin genes potentially giving rise to several different actins. We have, therefore, determined the complete amino acid sequence of actin from vegetative Dictyostelium cells. This sequence is unique and agrees with the DNA sequences of four actin genes for that region of the DNA, which is currently known. The protein sequence does not agree with the three other 'genes' and we discuss the possible expression of minor actin species.

Immunological differences between actins from cardiac muscle, skeletal muscle, and brain

The antigenic similarities and differences between various actins were explored by use of antisera against purified bovine cardiac actin and chicken embryo brain actin. In double-antibody coprecipitation tests, purified iodinated actins from bovine cardiac muscle, rabbit skeletal muscle, chicken embryo brain, and bovine brain all bound to antiserum against chicken embryo brain actin. This result demonstrates the presence of shared antigenic determinants among these actins. Cardiac actin antiserum, on the other hand, bound cardiac and skeletal actin, but failed to bind significantly either brain actin. In radioimmunoassay, all four unlabeled actins were capable of some degree of inhibition of binding of (125)I-labeled chicken embryo brain actin to homologous antiserum. The results confirm the existence of shared or similar antigenic determinants, but also show that the molecules are not antigenically identical. In the cardiac actin radioimmunoassay, unlabeled cardiac and skeletal muscle actins inhibited the binding of (125)I-labeled cardiac actin to homologous antiserum, but neither brain actin inhibited the binding. Thus, the muscle actins possess at least one antigenic determinant not expressed by the brain actins, in addition to the shared determinants. Furthermore, cardiac actin and skeletal actin generated different inhibition curves in the cardiac actin radioimmunoassay, demonstrating that, although antigenically related, they are not identical. Correlations with existing sequence data imply that substitutions in only a few residues alter the antigenic properties of actin.

Molecular cloning of the actin gene from yeast Saccharomyces cerevisiae

Two overlapping DNA fragments from yeast Saccharomyces cerevisiae containing the actin gene have been inserted into pBR322 and cloned in E.coli. Clones were identified by hybridization to complementary RNA from a plasmid containing a copy of Dictyostelium actin mRNA. One recombinant plasmid obtained (pYA102) contains a 3.93-kb Hindlll fragment, the other (pYA208) a 5.1-kb Pstl fragment, both share a common 2.2-kb fragment harboring part of the actin gene. Cloned yeast actin DNA was identified by R-loop formation and translation of the hybridized actin mRNA and by DNA sequence analysis. Cytoplasmic actin mRNA has been estimated to be about 1250 nucleotides long. There is only one type of the actin gene in S.cerevisiae.

Actin nascent chains are substrates for cyclic AMP-dependent phosphorylation in vivo

Two-dimensional gel electrophoresis of extracts of S49 mouse lymphoma cells labeled with [35S]methionine in the presence of inducers or analogs of cyclic AMP reveals a protein that both affinity purification and peptide mapping show to be a form of nonmuscle actin. This actin species also exhibits cyclic AMP-dependent labeling with [32P]phosphate, and, after acid hydrolysis, 32P label is found associated with phosphoserine. Phosphorylated actin does not appear when cells prelabeled with [35S]methionine are treated with an inducer of cyclic AMP in the presence of emetine, an inhibitor of protein synthesis; this suggests that only the nascent form of actin is a substrate for cyclic AMP-dependent phosphorylation. As well as differing slightly in isoelectric points, beta and gamma actins are found to yield different partial proteolytic cleavage products with staphylococcal protease. This microheterogeneity in the major cellular actin component is repeated in both the metabolically labile delta/epsilon actin and phosphorylated actin, suggesting that these three forms of actin derive from the same two gene products.

Construction of recombinant plasmids containing rat muscle actin and myosin light chain DNA sequences

The construction and partial characterization of recombinant bacterial plasmids carrying DNA sequences that hybridize with rat skeletal muscle actin and a myosin light chain mRNA is described. DNA of one clone hybridizes specifically with the muscle-specific alpha-actin mRNA. Three plasmid clones contain DNA inserts that hybridize with muscle as well as with nonmuscle actin mRNA. A fifth plasmid contains sequences complementary to mRNA coding for myosin light chain 2. DNA of this plasmid hybridizes specifically with RNA extracted from muscle and differentiated muscle cultures but not with RNA extracted from proliferating mononucleated myogenic cells.

The complete amino acid sequence of actins from bovine aorta, bovine heart, bovine fast skeletal muscle, and rabbit slow skeletal muscle. A protein-chemical analysis of muscle actin differentiation

Complete amino acid sequences for four mammalian muscle actins are reported: bovine skeletal muscle actin, bovine cardiac actin, the major component of bovine aorta actin, and rabbit slow skeletal muscle actin. The number of different actins in a higher mammal for which full amino acid sequences are now available is therefore increased from two to five. Screening of different smooth muscle tissues revealed in addition to the aorta type actin a second smooth muscle actin, which appears very similar if not identical to chicken gizzard actin. Since the sequence of chicken gizzard actin is known, six different actins are presently characterized in a higher mammal. The two smooth muscle actins--bovine aorta actin and chicken gizzard actin--differ by only three amino acid substitutions, all located in the amino-terminal end. In the rest of their sequences both smooth muscle actins share the same four amino acid substitutions, which distinguish them from skeletal muscle actin. Cardiac muscle actin differs from skeletal muscle actin by only four amino acid exchanges. No amino acid substitutions were found when actins from rabbit fast and slow skeletal muscle were compared. In addition we summarize the amino acid substitution patterns of the six different mammalian actins and discuss their tissue specificity. The results show a very close relationship between the four muscle actins in comparison to the nonmuscle actins. The amino substitution patterns indicate that skeletal muscle actin is the highest differentiated actin form, whereas smooth muscle actins show a noticeably cloer relation to nonmuscle actins. By these criteria cardiac muscle actin lies between skeletal muscle actin and smooth muscle actins.

Multiple actins in Drosophila melanogaster

The tissue and developmental specificities of the three Drosophila isoactins, originally identified in primary myogenic cultures and in the permanent Schneider L-2 cell line, have been investigated. Of these three isoactins (I, II, and III), actins I and II are stable and actin III is unstable. Two-dimensional polyacrylamide gel electrophoretic analyses of total cellular extracts after 1-h [(35)S]methionine pulses were performed on a large variety of embryonic, larval, and adult muscle and nonmuscle tissues. The results suggest that isoactins II and III are generalized cellular actins found in all drosophila cell types. Actin I, on the other hand, is muscle-associated and is found exclusively in supercontractile muscle (such as larval body wall and larval and adult viscera) including primary myogenic cell cultures. Although actin I synthesis is not detectable during very early embryogenesis, it is detectable by 25 h and actin I is a major stable actin in all larval muscle tissues. Actin I is synthesized in reduced amounts relative to the other actins in late third instar larvae but is again a major product of actin synthesis in the adult abdomen. A stable actin species with the same pI as actin III has been identified in the adult thorax and appears to be unique to flight muscle tissue. This new stable form of thoracic actin may be the result of a stabilization of the actin III found in other tissues or may be an entirely separate gene product.

Localization of the charge differences in the actins of rabbit skeletal muscle and chicken gizzard by two-dimensional gel electrophoretic analysis of tryptic fragments

Partial tryptic cleavage products of pure actin from rabbit skeletal muscle and chicken gizzard are compared by two-dimensional electrophoresis in polyacrylamide gels with respect to isoelectric point and molecular weight. While the intact polypeptides (Mr 42,000) have different isoelectric points, two large cleavage products (Mr 35,000) generated from both both actin species have identical isoelectric points and identical molecular weights. These relatively trypsin-resistant cleavage products are presumably identical to the known "core actin" fragments which lack the aminoterminal region of the polypeptide chain. Therefore the differences that are responsible for the different isoelectric points of rabbit skeletal muscle actin and chicken gizzard actin seem to be restricted to the aminoterminal part of the actin polypeptide chains as was proposed on the basis of partial amino acid sequence data.

Actin in mammalian lens

In this paper evidence is provided that one of the protein components of the water-soluble fraction of the calf lens binds specifically to deoxyribonuclease I (DNAse I). On the basis of this property, the polypeptide could be purified by applying DNAse I affinity chromatography. Concomitantly a protein of Mr55000 and a rather large amount of alpha-crystallin copurify with this polypeptide, which has a molecular weight of 42000. Highly purified 42000-Mr protein was also obtained by extraction of the water-insoluble fraction of the calf lens with 2-([tris(hydroxymethyl)methyl]amino) ethanesulfonic acid followed by gel filtration. Amino acid analyses, peptide mapping and electron microscopy show that the protein obtained from both lens fractions is identical to non-muscle actin. Furthermore the amino acid composition of the 55000-Mr protein is identical to hog stomach skeletin and very similar to calf brain desmin.

Immunofluorescent visualization of 100 A filaments in different cultured chick embryo cell types

Antibody prepared against the 55,000 dalton subunit of reconstituted chick gizzard 100 A filaments (anti-G55K) bound to the 100 A filaments of chick smooth muscle, cardiac muscle, and skeletal muscle cells, and to the 100 A filaments of Schwann cells and satellite glial cells of the peripheral nervous system. Anti-G55K did not bind to replicating presumptive myoblasts, fibroblasts, chondroblasts, pigment cells, neurons, or to central nervous system glial cells. This contrasted with the wider range of binding of antibody to the 58,000 dalton subunit of chick fibroblast 100 A filaments (anti-F58K) which bound to the 100 A filaments of all cell types examined except hepatocytes and skin epithelial cells. Anti-G55K) staining revealed a morphologically distinct distribution of 100 A filaments in the three types of muscle cells. Spindle shaped smooth muscle cells exhibited dense fluorescent staining near the poles of the cells, and also exhibited unique patches of fluorescent material after cytochalasin B and Colcemid treatment. In myotubes, the fluorescence was limited to longitudinal bundles of filaments between the striated myofibrils. Cardiac cells contained uniformly distributed fine filaments. Lastly, smooth muscle cells in various phases of mitosis bound the anti-G55K, whereas replicating presumptive skeletal myoblasts failed to bind the anti-G55K.

Actin amino-acid sequences. Comparison of actins from calf thymus, bovine brain, and SV40-transformed mouse 3T3 cells with rabbit skeletal muscle actin

Actin was purified from calf thymus, bovine brain and SV40-transformed mouse 3T3 cells grown in tissue culture. Isoelectric focusing analysis showed the presence of the two actin polypeptides beta and gamma typical for non-muscle actins in all three actins. Tryptic and thermolytic peptides accounting for the complete amino-acid sequence of the cytoplasmic actins were separated and isolated by preparative fingerprint techniques. All peptides were characterized by amino-acid analysis and compared with the corresponding peptides from rabbit skeletal muscle actin. Peptides which differed in amino-acid composition from the corresponding skeletal muscle actin peptides were subjected to sequence analysis in order to localize the amino-acid replacement. The results obtained show that all three mammalian cytoplasmic actins studied contain the same amino-acid exchanges indicating that mammalian cytoplasmic actins are very similar if not identical in amino-acid sequence. The presence of two different isoelectric species beta and gamma in cytoplasmic actins from higher vertebrates is acccounted for by the isolation of two very similar but not identical amino-terminal peptides in all three actin preparations. The nature of the amino-acid replacements in these two peptides not only accounts for the different isoelectric forms but also shows that beta and gamma cytoplasmic actins are the products of two different structural genes expressed in the same cell. The total number of amino-acid replacements so far detected in the comparison of these cytoplasmic actins and skeletal muscle actin is 25 for the beta chain and 24 for the gamma chain. With the exception of the amino-terminal three or four residues, which are responsible for the isoelectric differences, the replacements do not involve charged amino acids. The exchanges are not randomly distributed. No replacements were detected in regions 18--75 and 299--356 while the regions between residues 2--17 and 259--298 show a high number of replacements. In addition documentation for a few minor revisions of the amino acid sequence of rabbit skeletal muscle actin is provided.

Actins from mammals, bird, fish and slime mold characterized by isoelectric focusing in polyacrylamide gels

Actins isolated from a variety of tissues and cultured cells were compared by isoelectric focusing in polyacrylamide gels in the presence of 9 M urea and 2% Nonidet P40. Actins isolated from muscle tissue with a sarcomeric structure like skeletal muscle and heart muscle invariably display, as previously shown, one single band with a pI of approximately 5.4 (alpha-actin) in isoelectric focusing gels. Actins isolated from mammalian or avian non-muscle tissue and cultured mammalian cells display two polypeptide bands (beta and gamma-actins) focusing at a slightly higher pH than alpha-actin as a closely spaced doublet. A gamma-like actin is the predominant species in chicken gizzard actin. However, this gamma-like form is not isoelectrically identical with gamma-actin from brain. These results are discussed in relation with the currently available amino acid sequence data known for different actins. Actin isolated from the liver of the electric fish Torpedo marmorata appears to consist of a single isoelectric species with an apparent isoelectric point similar to the beta-actin component of mammalian brain. The actin from the slime mold Physarum polycephalum shows only one single major band in isofocusing gels with an isoelectric point lower than that of alpha-actin.